The present invention relates generally to cytokine receptors, and more specifically, to Interleukin-15 receptors.
Interleukin-15 (IL-15) is a recently identified cytokine with biological activities similar to IL-2 (Grabstein et al., Science 264:965, 1994). There is approximately 96% nucleotide sequence identity and 96% amino acid sequence identity between human and simian IL-15, and approximately 81% nucleotide sequence identity and 73% amino acid sequence identity between human and murine IL-15.
Northern analysis of a variety of human tissues indicated that IL-15 mRNA is expressed by many human tissues and abundantly by placenta and skeletal muscle. Significant levels of IL-15 mRNA were also observed in other tissues including kidney, lung, liver, and heart. The best sources of IL-15 mRNA so far observed have been adherent mononuclear cells (monocyte enriched, PBM) and epithelial and fibroblast cell lines such as CV-1/EBNA and IMTLH. Activated peripheral blood T cells (PBT), a rich source of IL-2, express no detectable IL-15 mRNA.
IL-15 shares many biological properties with Interleukin-2 (“IL-2”). These properties include proliferation and activation of human and murine T cells and the generation of lymphokine activated killer cells (LAK), natural killer cells (NK) and cytotoxic T lymphocytes (CTL). IL-15 also can co-stimulate with CD40 ligand (CD40L) proliferation and immunoglobulin secretion by B lymphocytes.
In view of the shared biological properties with IL-2, tests were conducted to determine whether IL-15 uses any of the components of the IL-2 receptor. IL-2 cell surface receptors (IL-2R) contain at least three subunits, α, β and γ (Toshikazu et al., Science, 257: 379 (1992); see also Minami et al., Annu. Rev. Immunol. 11, 245, 1993, for a recent review). The β and γ chains are required for high affinity IL-2 binding and IL-2 signaling and are members of the hematopoietin receptor superfamily. The α chain (or p55) is a low affinity, non-signaling binding subunit, and the only cytokine receptor member of a large family of binding proteins whose members include complement receptor proteins (Perkins et al., Biochemistry 27:4004, 1988; Davie et al., Cold Spring Harb. Symp. Quant. Biol. 51:509, 1986). The γ chain of the IL-2R has been shown recently to be shared by receptors for several other cytokines (IL-4, IL-7, IL-9; (Noguchi et al., Science 262:1877, 1993; Kondo, et al., Science 262:1874, 1993; Kondo et al., Science 263:1453, 1994; Russell et al., Science 262:1880, 1993; Russell, et al., Science 266:1042, 1994) and designated the common γ chain or γc.
Several lines of evidence suggest that there is an IL-15 specific binding protein. For example, an IL-3 dependent murine cell line, 32D (J. S. Greenberger et al., Fed. Proc. 42: 2762 (1983)), expressed the complete IL-2R and proliferated in response to IL-2, but cannot bind or respond to IL-15 (Grabstein et al., supra). Similarly, early murine pre-T cells derived from day 13 fetal liver that lack CD3, CD4 and CD8 expression (triple negative, or TN, cells) expressed all three IL-2R subunits, proliferated in response to IL-2, but did not bind or respond to IL-15 (Giri et al., EMBO J. 13:2822, 1994). On the other hand, certain human cell types and cell lines (e.g., umbilical vein endothelial cells, fibroblasts and thymic and stromal cells) did not bind IL-2 but bound IL-15 with high affinity (Giri et al., supra).
Additionally, antibodies directed against the α chain of the IL-2 receptor (anti-IL-2Rα) have no effect on IL-15 (Grabstein et al., supra; Giri et al., supra). Antibodies directed against the IL-2Rβ, however, are able to block the activity of IL-15, suggesting that IL-15 uses the B chain of IL-2R. Similarly, some cells require the γ chain of IL-2R for IL-15 signal transduction (Giri et al., supra) IL-15 requires the p chain of the IL-2R for all the biological activities tested, but the α chain of the IL-2R is not required (Giri et al., supra; Grabstein et al., supra). However, prior to the present invention, neither an IL-15-specific binding protein, nor a DNA encoding such protein, had been isolated.